Modern backplanes, also referred to as motherboards, serve as a communication medium for the exchange of electronic signals between a plurality of daughter cards. Each daughter card generates communication signals, for example data bus signals, address bus signals, and control signals. The signals are distributed to connectors mounted along a side of the daughter card. The daughter card connectors mate with a corresponding set of connectors on the backplane for providing interconnect and distribution of the signals between daughter cards along various communication paths on the backplane.
A chassis houses the backplane, daughter cards, and corresponding connectors. Backplane connectors are spaced in equidistant rows along the backplane, each row defining a card slot. Card guides mounted along the side of the chassis guide the daughter cards into alignment with corresponding backplane card slots. This assures proper mating of the daughter card and backplane connector pairs in each slot.
Each connector pair includes a plurality of interconnects in the form of conductive mating pins and terminals which couple by frictional contact. The plastic connector units housing the mating pins and terminals also exhibit inherent coupling friction. Consequently, a certain amount of coupling force is required to overcome the coupling friction for mating each connector. With several connectors on each daughter card, all simultaneously inserted into corresponding backplane connectors in a card slot, this coupling force can add up to a significant amount. For example, some connector configurations can require 80-100 ft-lbs of force for joining a daughter card to a backplane. The coupling force increases as the interconnect count increases.
Extractors and inserters mounted on an edge of the daughter card opposite the connector edge are often employed to serve as levers for generating the requisite coupling force. Any coupling force applied to the daughter card translates directly through the card slot connectors to the backplane. The side edges of the backplane are bolted to the chassis with bolts. With coupling force applied to the center of the backplane by the daughter card, and an equal and opposite force applied to the edges of the backplane by the chassis, the backplane is subject to a torque or bending moment. This in turn causes a deflection or bowing of the backplane. Bowing is undesirable because any bending moments may cause damage to the surface and/or inner layers of the backplane. In addition, any play in the backplane can inhibit proper mating between the daughter card and backplane connectors.
To mitigate the effects of deflection, backplanes often include structures for resisting longitudinal and lateral flexibility. Longitudinal backplane rigidity, along the major backplane axis perpendicular to the card slots, is generally provided by the backplane mounting bolts used for mounting the backplane to the chassis. Lateral backplane rigidity along the minor backplane axis parallel to the card slots, is often provided by rigidity enhancers laterally or longitudinally disposed across the center of the backplane. These rigidity enhancers have taken on various forms in the past.
A popular embodiment employs extrusion bars mounted laterally and/or longitudinally across the backside of the backplane and fixed to the backplane chassis. This provides a rigid support structure to resist backplane deflection. In backplane configurations with densely-populated components, extrusion bars interfere with the components. To overcome this problem, standoffs are used to distance the extrusion bar from the backplane such that the extrusion bar suspends over the components between contact points. Another popular method involves increasing the thickness of the backplane by adding backplane layers or by increasing the thickness of each individual layer. A thicker backplane is inherently more rigid.
Modern backplane configurations require ever-increasing daughter card interconnect capabilities due to wider communication busses and increased data throughput. This results in increased connector density, with higher connector pin counts, placing further demands on backplane rigidity. In some backplane configurations, the backplane surface is populated with connectors and surface-mount components to the extent that standard laterally-oriented stiffeners and/or extrusion bars cannot be applied to the backplane in an economically feasible manner. As an example, the VME 64 Extension backplane configuration includes three connectors of high pin density in each card slot. To further increase interconnect, the space between the outer and middle connectors in each card slot is minimized and consequently will not accommodate a longitudinally-disposed stiffener structure of sufficient strength.
To complicate matters, the middle connector and outer connectors are laterally offset from each other in some configurations, for example VME 64 Extension configurations. Offset connector arrangements can result from the connectors being of various widths, or from the connectors being spatially shifted within a card slot. In any event, such offset configurations result in a non-linear "dog-legged" channel between rows of connectors. The term "dog-legged" as used herein is intended to mean a channel, one or more portions of which are offset, displaced, or misaligned with respect to other portions, i.e. a channel including an abrupt bend, in which one channel section is turned out of line, but nearly parallel with other channel sections. "Dog-legged" also refers to a portion of a stiffener interposed in such a channel. Such "dog-legged" channels prevent insertion of standard prior art linear stiffeners and extrusion bars. To date, the problem of stiffening a backplane having "dog-legged" connector channels has not been addressed, other than stiffening by adding thickness to the construction of the backplane, which in turn increases manufacturing costs.